Wireless communication systems, including cellular phones, paging devices personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCs systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user's total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service and increasing capacity in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations and other access terminals (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data on the same frequency in assigned channels that correspond to specific unique orthogonal codes. For example, a mobile station may receive forward channel data signals from a base station that are encoded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are framed, repeated, encoded, block interleaved, modulated by a 64-ary orthogonal modulation, and spread prior to transmission by the mobile station. In another embodiment, a base station may receive reverse channel data signals from the mobile station that are framed, repeated, encoded, block interleaved, spread prior to transmission by the mobile station. Those skilled in the art will recognize that mobile station may employ quadrature phase shift keying (QPSK) modulation, binary phase shift keying (BPSK) modulation, quadrature amplitude modulation (QAM) or other digital modulation format for modulation of an RF carrier for transmission of the data signals. One such implementation is found in the T1A IS-95 CDMA standard. Another implementation is the TIA IS-2000 standard.
It is preferable to implement wireless CDMA cellular or PCS systems that take full advantage of the performance improvements offered by adaptive antenna technologies. These improvements include increased range or cell size, reduced interference, and greater cell capacity. However, a constraint on system performance with adaptive antenna techniques is the range limitation of the pilot channel, synchronization (sync) channel, and paging channel overhead signals, since uniform sector coverage is required for these signals. This coverage requirement excludes the use of directive antenna gain offered by narrow transmission beams of adaptive antennas. For transmission of the pilot channel, sync channel and paging channel signals, a limited range implies that a base station transceiver might not make full use of the capacity offered by adaptive antenna techniques, depending on the distribution of access terminals. Full range and capacity coverage may require the application of more power in the overhead channels, which results in less available power to support subscriber traffic.
Therefore, there is a need in the art for CDMA wireless devices that improve the detection of CDMA pilot channels signals in order to extend the effective range and coverage of the base station transmitter. In particular, there is a need for CDMA mobile stations and access terminals that are capable of detecting pilot channel signals having a low chip signal-to-noise ratio (Ec/Io). More particularly, there is a need for an improved detection method that reduces the power level required for the pilot channel, which allows the assignment of more power in the base station to traffic channels.